This invention relates generally to magnetic flowmeters, and more particularly to a flowmeter in which the fluid to be metered is conducted through a pipe to intersect a magnetic field, the flowmeter including electrodes in the region of the magnetic field which are capacitively coupled to the fluid to yield a signal that is a function of flow rate.
German patent publication No. 1963901 discloses a magnetic flowmeter whose electrodes are in contact with the fluid being metered and are therefore galvanically coupled thereto. With this electrode arrangement, it is possible to derive signals that are essentially proportional to the flow rate even if the flow profile is non-axisymmetric. To this end, the magnetic field is accommodated to the weighting factors. One must, however, recognize that it is theoretically impossible to fully attain this result, as a consequence of which flow errors arise depending on the flow profile.
Moreover, since in this German patent publication, the electrodes which are galvanically coupled to the fluid are rectangular in form, they are subject to contamination by the fluid. This contamination can result in erroneous readings; for the signal from the contaminated electrodes is no longer proportional to flow rate.
The disadvantages which arise in an arrangement in which the flowmeter electrodes are in contact with the fluid are eliminated in the electrode arrangement disclosed in German patent DE-PSI No. 548,918 in which the electrodes are capacitively coupled to the fluid. However, this patent is silent on the magnetic field distribution and the electrode geometry necessary to minimize or eliminate the adverse influence of flow profile on the electrode signal.
Also known are magnetic flowmeters which include multiple-point electrodes in which weighted signals from the electrodes are added in a converter. This multiple-point electrode arrangement makes necessary a more complicated construction, particularly with regard to sealing problems. It also entails more complex electronic circuits.